Spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS) are neurodegenerative disorders of unknown etiology. They have in common death of lower motor neurons (LMN) causing muscle weakness, and both disorders are fatal. Mechanisms of LMN death differ for SMA and ALS. In SMA, LMN death may occur over a limited period of time. Unanswered is whether there is late or continued LMN loss. Recent genetic studies in SMA indicate a relationship between survival motor neuron gene (SMN2) copy number and SMA type. Unanswered is the relationship between copy number and LMN number. In ALS, no single mechanism of LMN death explains known features, and a cascade of events ultimately leading to LMN death is likely. Unanswered in ALS is the natural pattern of progression of LMN loss from muscle to muscle. Although muscle weakness is the clinical manifestation of LMN loss for both disorders, the rate of loss of strength does not accurately reflect the rate of loss of LMNs. The discrepancy is due to the compensatory effects of reinnervation of denervated fibers by collateral sprouting from surviving motor nerve terminals. Similarly, routine electrophysiologic tests do not accurately measure LMN loss. Unanswered for both disorders is the dynamics of the compensatory process that determines the clinical state and level of function. Motor unit number estimation (MUNE) is a special electrophysiologic test that can directly assess the number of LMNs innervating a muscle. There are no data on the natural course of LMN loss for SMA, and little data for ALS. We propose to develop and refine MUNE and other electrophysiologic techniques to study, and follow the course of LMN loss and associated compensatory changes. For SMA, we will adapt MUNE techniques to study infants and children. For older SMA and ALS, we will refine MUNE techniques to optimize data collection. For SMA, we will correlate LMN loss with clinical type and SMN2 copy number. We will begin, in the two years of the grant-performing serial studies, to assess whether there is continued LMN loss. For ALS, we will determine and compare the rate and pattern of LMN loss in distal and proximal muscles. In older SMA and ALS, we will assess relationships between LMN loss and measures of collateral reinnervation and strength. We anticipate that MUNE and other electrophysiologic techniques will have direct applicability to the design of clinical trials for SMA and ALS, because these techniques can be used as informative end-point measures. To facilitate the use of MUNE in clinical trials, we will develop and refine the techniques in a form that can be used in any clinical center participating in trials. Currently, most MUNE techniques rely on proprietary software. We will develop software for use on PC-based computer systems, making them available to all laboratories.